1. Field of the Invention
The present invention relates to a transmission power control method of a spread-spectrum communication system, and a spread-spectrum communication system employing the control method, which can be preferably applied to the transmission power control in cellular CDMA (Code Division Multiple Access) systems in radio communications.
2. Description of Related Art
As is well known, since a plurality of users share the same frequency band in a CDMA system, signals from the other users interfere with the signal of a user, thereby degrading the communication quality of the user. In addition, when a first mobile station near a base station and a second mobile station faraway from the base station perform communications at the same time, a transmitted signal from the first mobile station will be received at higher power by the base station, and a transmitted signal from the second mobile station will be received at lower power.
Thus, a near-far problem arises in that channel quality will be greatly degraded in communications between a base station and faraway mobile stations owing to the interference from mobile stations near the base station. In view of this, transmission power control has been studied as a technique for overcoming the near-far problem. The transmission control is carried out by controlling transmission power in such a manner that the received power by a receiving station, or the SIR (Signal-to-Interference power Ratio) obtained from the received power is kept constant independently of the locations of a mobile station, thus providing uniform channel quality throughout the service area. In particular, in reverse (from mobile station to base station) channels, the transmission power control of respective mobile stations is performed such that the received power levels of signals transmitted from the mobile stations and received by the base station, or the SIRs associated with the received power levels are kept constant at the base station.
In particular, in the CDMA system which considers the interference from the other users as white noise, equivalent noise power increases with the number of users, and hence, the capacity in terms of the number of subscribers in the cell is determined on the basis of the received SIR which can maintain predetermined channel quality.
On the other hand, in forward (from base station to mobile station) channels, the received SIR is kept constant because the intended channel signal travels through the same propagation paths as the signals for the other users which cause the interference, and undergoes the same fluctuations as the interference waves, where the fluctuations include long-term, short-term, and instantaneous fluctuations. Therefore, the transmission power control is not required when handling only the interference in the same cell.
The CDMA system, which handles the interference as white noise, however, must take account of the interference from other cells because it shares the same frequency band with adjacent cells. Although the interference power from other cells takes a form of instantaneous fluctuations due to Rayleigh fading as the interference power in the same cell, the fluctuations differ from those of the desired signal. According to the CDMA system standard by TIA (Telecommunications Industry Association) , no forward transmission power control is performed basically, except when a frame error rate at a base station exceeds a predetermined threshold level, in which case the transmission power of the base station to the mobile station is increased. This is because a large amount of transmission power change will increase the interference to other cells. The transmitted signals from the base stations of other cells, however, become instantaneously fluctuating interference to an intended channel, and this conventional system cannot follow the instantaneous fluctuations.
As a conventional transmission power control method which can track the instantaneous fluctuations, a transmission power control method based on a closed loop control using a transmission power control bit is known.
FIGS. 1A and 1B show examples of the transmission power control method based on the closed loop control. As shown in FIGS. 1A and 1B, when a mobile station performs communications with a base station within the cell, the mobile station measures the received power of the desired signal from the base station, and determines a transmission power control bit for controlling the transmission power of the base station on the basis of the measured result (steps S1-S4). The mobile station inserts the transmission power control bit into the signal to be transmitted, and transmits it to the base station. The base station receives the signal transmitted from the mobile station, extracts the transmission power control bit, and determines its transmission power in accordance with the transmission power control bit (steps S5 and S6).
Likewise, the base station measures the received power of the desired signal from the mobile station, and determines a transmission power control bit for controlling the transmission power of the mobile station on the basis of the measured result (steps S11-S14). Then, the base station inserts the transmission power control bit into the signal to be transmitted, and sends it to the mobile station. The mobile station receives the signal transmitted from the base station, extracts the transmission power control bit, and determines its transmission power in accordance with the transmission power control bit (steps S15 and S16).
According to the conventional closed loop transmission power control method described above in reference to FIGS. 1A and 1B, the insertion interval of the transmission power control bit must be shorter than the Doppler fluctuation period (=1/Doppler frequency) in order to absorb the instantaneous fluctuations due to Rayleigh fading. For example, when a carrier of 2 GHz band is used by a mobile station moving at 60 km/h-70 km/h, the Doppler frequency becomes about 200 Hz. Accordingly, the transmission power control bit must be inserted into a frame at every few millisecond interval.
On the other hand, taking account of frame efficiency (transmission efficiency), the number of transmission power control bits per transmission power control is limited to 1-2 bits. Furthermore, a controlled amount is usually set small to achieve a high accuracy transmission power control. Therefore, the conventional system cannot follow a sudden changes in the received power. In particular, since there are many high buildings in urban areas, the propagation path of a mobile station may suddenly be transferred from a shadow of a high building to a line of sight area, or vice versa. In such cases, the received signal level at the base station will vary by more than 30 dB.
In the case where the transmission power control in the reverse direction operates normally as described above, the base station received powers (or SIRs) of the signals transmitted from respective mobile stations become constant, and hence, uniform receiving quality can be obtained.
However, when a mobile station suddenly moves out of a shadow of a building to a line of sight area, the base station's received power of the signal transmitted from the mobile station suddenly increases, which induces large interference to signals transmitted from the other mobile stations. In such cases, the transmission power control bit of a small controlling quantity cannot quickly reduce the transmission power. This presents a problem in that large interference to the other users takes place, and the capacity in terms of the number of subscribers is reduced.